The present invention relates generally to electric motors, and more specifically to rotors having high energy rare earth magnets. This invention controls cogging and ripple torque. It uses a rotor which eliminates loss of magnet material and permits use of a more easily wound stator.
U.S. Pat. No. 3,979,821, entitled "Method of Manufacturing Rare Earth Permanent Magnet Rotor", issued Sept. 14, 1976, discloses a rotor containing rare earth magnet material which can be assembled, machined and then magnetized. A stack of laminations is built up, pieces of virgin rare earth magnet material are inserted in the slots provided therefor in the laminations, the laminations with the rare earth magnet material are cast with aluminum, the cast rotor is machined and the rare earth magnet material is then magnetized.
Since the brushless motors of the type described in the above referenced patent typically include highly magnetic rare earth magnets, they are particularly sensitive to an objectionable cogging torque. Cogging torque is primarily caused by the magnetic attraction of stator teeth to uniform edges of concentrated magnetic positions on the surface of a rotor. Cogging causes variations in motor torque and speed and may prevent a motor from being used in precision servo applications. The conventional correction for this situation is to skew the stator one slot pitch for stators having an even number of slots or one half slot pitch for stators having an odd number of slots. However, in many cases it is difficult or impossible to properly insert the windings in the skewed slots.
An alternative solution to the problem of cogging is disclosed in an article entitled "Brushless Servomotors Use Notched Magnet to Stop Cogging," Electronic Design, Nov. 11, 1982. The alternative solution requires that a "V" shaped notch be cut into the magnet itself. Although this may be practical in some magnets, cutting a notch in a rare earth magnet is particularly difficult, since the magnets break easily during machining. Accordingly, there is a need for an arrangement which prevents cogging but which does not require skewed slots or notches in the magnets.
It is an object of the present invention to reduce cogging torque to a minimum in a motor having a permanent magnet rotor and wound stator.
It is another object of the present invention to reduce cogging without skewing the slot pitch of the stator and without cutting notches in the permanent magnets.